Early studies, beginning with those of Stich et al. in 1959, of spontaneous leukemias in AKR mice revealed that a substantial proportion of cells had 41 chromosomes. With the advent of trypsin-Giemsa banding techniques for chromosome identification, we recently reinvestigated karyotypes of AKR thymomas. Most of those cells with 41 chromosomes were trisomic for a specific chromosome, number 15. Additional chromosomes and/or chromosome 15 trisomy were not found in the thymus of young mice or in pooled AKR embryo cells. We propose to investigate the possible significance of the specific karyotypic alteration to leukemogenesis in the AKR mouse initially by 3 approaches: 1) Chromosome banding techniques will be used to analyze more extensively those thymomas with modal chromosome numbers of 40 and also leukemic lymph nodes, in an attempt to determine the possible relationship between chromosome trisomy and leukemic progression. Leukemia cells from inbred strains (C58/J, C3H/Fg, PL/J, and irradiated C57BL/6) with high incidence spontaneous or induced leukemias will be analyzed; 2) Methodology for in situ hybridization of nucleic acids with chromosomes will be developed. Initial studies will involve ribosomal RNA since chromosome 15 is reported to have a secondary constriction (nucleolar organizing region) and to carry ribosomal DNA sites. Data will be interpreted in conjunction with quantitation of ribosomal RNA per cell as well as ribosomal DNA, by in vitro saturation hybridization, for nonleukemic, leukemic nontrisomic, and leukemic trisomic cells; 3) Chromosomal findings will be correlated with expression of serologically demonstrable alloantigens (TL, Thy-1, Ly-1, Ly-2, H-2k) on the surface of leukemia cells in individual thymomas.